1. Field of the Invention
The present disclosure relates generally to a method for inhibiting corrosion of alloys by surface treatment employing electrochemistry. More particularly, the present disclosure is directed to inhibiting the corrosion of alloys by treating the surface of the alloy with a salt of one or more elements of the rare earth group employing electrochemistry.
2. Description of the Related Art
In general, highly alloyed metals such as, for example, stainless steel alloys, are ordinarily utilized in environments subjected to corrosion conditions due to their resistance to pitting and crevice corrosion. Corrosion typically occurs in an environment where the alloys are in contact with an aqueous medium such as seawater, well water, saltwater and tap water contaminated with, for example, chloride. Examples of the various environments, where alloys are used include the off-shore industry (seawater, acid oil and gas), for heat exchangers and condensers (seawater), for desalination plants (saltwater), for flue-gas purification equipment (chloride-containing acids), for flue-gas condensing apparatus (strong acids), for plants for the production of sulphurous acid or phosphoric acid, for pipes and apparatus for oil and gas production (acid oil and gas), for apparatus and pipes in cellulose bleaching plants and in chlorate production plants (chloride containing, oxidizing acids or solutions, respectively) and for tankers and petrol trucks (all kinds of chemicals). The reason the stainless steel possesses such corrosion resistance is the high alloy content, which is believed to inhibit the corrosion processes. One such alloying element that provides the excellent corrosion resistance of these stainless steels is chromium because it forms a chromium oxide passive film on the surface of the steel. Other alloying elements, which also assist in improving the pitting corrosion resistance, are molybdenum and nickel.
Pitting corrosion is the first stage toward more serious forms of corrosion such as, for example, fatigue, stress corrosion cracking and hydrogen embrittlement in the alloy. Thus, it is important to inhibit pitting corrosion at the earliest stage possible. One way to enhance the corrosion resistance of alloys such as stainless steel alloys and, therefore, inhibit pitting corrosion is to dissolve corrosion inhibitors in the liquid that is in contact with the stainless steel structure. Another example to enhance the corrosion resistance is to add the corrosion inhibitors to a paint or polymer coating and then applying the paint or coating to the stainless steel structure.
Yet another example to increase the corrosion resistance of alloys is to provide a corrosion-resistant layer on the surface of the stainless steel alloy by incorporating cerium or other rare earth cations into the oxide film on the stainless steel's surface. This has been accomplished by immersing the steel into a solution of a cerium salt and water and then heating the solution to a high temperature. However, heating may not always be an option to incorporate the cerium and/or other rare earth ions on the surface of the alloy. For example, a structure made from the alloy may be part of an environment that may not tolerate heat or the water vapor that results from heating the solution containing rare earth salt. There may also not be a provision to capture the water vapor in an efficient manner. Accordingly, the surface of the alloy may lose its corrosion protection after a period of time resulting in an additional treatment of “corrosion proofing”.
Thus, it would be desirable to increase the corrosion resistance of alloys such as stainless steel alloys by introducing a salt of one or more rare earth elements, e.g., cerium, into the surface of the alloy without having to perform a high temperature step. The electrochemical treatment described herein provides such a step that is free of high temperature heating.